Converter circuit arrangements of this generic type are described, for example, in European Patent Applications EP 0 682 402 and EP 0 758 161, as well as in German Laid-Open Specification DE 196 07 201 A1. Such a converter circuit arrangement has a first converter, which can be connected, preferably via a transformer, to a three-phase voltage supply network. The first converter converts the AC voltage into a DC voltage but, at the same time, can also allow power to flow in both directions. The DC voltage supplies a DC intermediate circuit, which is essentially formed by a capacitor bank. The DC intermediate circuit is connected to a second converter which drives a load, for example a motor. The motor is, in particular, a rotating-field machine. Nowadays, frequency converters with a DC intermediate circuit (U converters) are generally used for driving rotating-field machines. Owing to the principle on which they operate, in addition to the desired balanced three-phase voltage, such converters also always produce an undesirable, but unavoidable, unbalanced, so-called common-mode voltage. This common-mode voltage produces a ground current, which is caused by the fact that the parasitic capacitances between the converter and ground (primarily caused by shielded cables) are continually harged and discharged by the common-mode voltage. The round current path is formed by: converter--ground point--ground conductor system--parasitic ground capacitances--converter. Modern systems are designed to have losses that are as low as possible. This therefore, of course, also applies to the ground current and leads to an oscillation being excited in the ground current path, and the current reaching a considerable amplitude. This represents a large additional load on the converter components. Furthermore, the superimposition of the ground current spikes on the balanced current triggers overcurrent protection thresholds. In order to avoid this, the entire converter would have to be highly overdesigned. In the past, this was not a problem since the semiconductor switches switched only very slowly, so that the additional losses were only minor. However, the problem has been exacerbated as the switching frequency and the switching flank gradient have increased. Thus, with the high switching frequencies that are possible nowadays, technical measures are required to attenuate the common-mode voltage. For this reason, in DE 196 07 201 A1, an interference protection filter is introduced into the intermediate circuit. The filter comprises a transformer and a set of filter capacitors. One winding of the transformer is connected in the positive path of the intermediate circuit, and the other in the negative path of the intermediate circuit, so that the unbalanced ground current magnetizes the transformer core, and the ground current is thus reduced.
The described converter arrangement is particularly suitable for applications in the low-voltage range, that is to say with rated voltages below 1 kV. It has the disadvantage that, in consequence, it is not possible to drive all types of rotating-field machines: The switching processes in the second converter lead to non-sinusoidal voltages at the motor terminals and, owing to the capacitive grounding via the filter capacitors, the common-mode voltage leads to voltage spikes between the motor and ground. In consequence, more complex motor designs are required, which can also be used with non-sinusoidal voltages.